Mixed Oxide Compounds Research Articles

Observation of CCNR-type electrical switching in Zn0.3Mn0.7+xSixFe2−2xO4 spinel ferrite series

A nonvolatile memory effect exhibited by electric field-induced resistance switching has been the topic of intense research not only due to its applications as resistive random access memory but also from the basic physics point of view. Among several binary and ternary mixed oxide compounds, the ones which possess magnetic ions have shown a great promise. Spinel ferrite system Zn0.3Mn0.7+xSixFe2−2xO4 with varying x is investigated for its novel electrical switching properties. Both temperature and applied voltage dependence of current-controlled negative resistance-type electrical switching showed better than 200% of resistive switching ratios. Bulk polycrystalline samples showed composition x dependence of resistive switching. The current–voltage characteristics are modeled for low and high applied field regime, and the presence of space-charge-limiting current is confirmed. Thin films of the ferrite system grown by pulsed laser deposition showed almost nonexistence of resistive switching, suggesting that the bulk composition of the compound has a major role to play against the film–electrode interface.

Actinide mixed oxide conversion by advanced thermal denitration route

In the framework of generation IV development for nuclear reactors, actinide mixed oxides are considered for multirecycling plutonium fuels and for transmutation targets of minor actinides. In this context, new processes are being developed for either the synthesis of mixed uranium-plutonium oxide compounds for MOx fuel or uranium-americium target fabrications. The main purposes are to simplify and step up industrial processes as well as to decrease actinide dust dispersion, and liquid effluent and gaseous releases. Among options for conversion route, a novel and patented advanced thermal denitration in presence of organic additives was established successfully to synthesize UO2+δ, U0.55Pu0.45O2±δ, and U0.9Am0.1O2-δ oxides. Here, we describe the different intermediate steps of this process together with the characterization of the oxides obtained. The data highlight several advantages of this new route for actinide conversion.

Study of the factors affecting the formation of copper–chromium/aluminum oxide compounds with a spinel structure

The effect of the Cr3+/Al3+ ratio on the crystallization temperature of mixed oxide compounds with a spinel structure and their structural features and morphological characteristics have been studied using a combination of physicochemical methods: thermal analysis, IR spectroscopy, X-ray powder diffraction, and electron microscopy. The role of temperature of synthesis and drying of Cu–Cr/Al hydroxy precursors in the formation of copper-containing spinels CuCrxAl2–xO4, where x = 0–2, has been elucidated. The results are of interest for selection of the optimal composition and conditions of synthesis and formation of copper-containing spinels for their practical use.

Heat treatment-induced compositional and structural changes in the preparation of CuCrAlO4 and CuFeAlO4

The formation of the spinel oxide compounds CuCrAlO4 and CuFeAlO4 has been studied at the nanolevel using thermal analysis, X-ray diffraction, high-resolution elemental microscopy, and differential dissolution. The results demonstrate that, in the case of the synthesis procedure used in this study, spinel structures are formed by solid-state reactions. We have studied the dynamics of changes in the chemical and phase compositions, lattice parameters, structure, and morphology of spinel particles at temperatures of 600 and 900°C. Our findings are of practical interest in adjusting conditions for the formation of mixed oxide compounds with the spinel structure.

Dilatometric study of a co-converted (U,Am)O2 powder

A possible option to reduce the radiotoxicity of nuclear waste is to transmute minor actinides and notably americium into lighter short-lived elements in fast neutron reactors of fourth generation. It consists in irradiating uranium–americium mixed-oxide compounds named AmBB (Americium-Bearing Blankets) and located at the reactor core periphery. Among the processes developed to fabricate AmBB, a chemical synthesis route was investigated. Dense samples were achieved using a simplified pelletizing-sintering process from oxalate co-converted powder. This work focuses on the identification and comprehension of the steps occurring during the sintering of these co-converted (U,Am)O2 powders. To discriminate the different phenomena observed by dilatometric measurements, different experimental techniques were performed. It allowed identifying that the sintering of the finest particles occurred in the [1000–1350K] range while the elimination of porosity and sintering of (U,Am)O2 larger grains is realized in the [1400–1900K] range with an apparent activation energy of 430kJ.mol−1.

Studies on electrochemical behavior of uranium species in choline chloride-urea eutectic for developing electrolytically treating method of uranium-bearing wastes

As one of methods for recovering uranium from the uranium-bearing wastes, we have proposed the electrolytic deposition method using choline chloride-urea (CCU) eutectic as media. Bulk electrolysis of the CCU solutions dissolving uranium-bearing wastes were carried out at −1.5 V at 80 °C. The deposits were formed on a carbon electrode as cathode and mainly consist of the U(VI) and U(IV) mixed oxide compounds containing F, O, and N. Consequently, it was confirmed that the CCU eutectic is effective media for recovering uranium selectively from uranium-bearing waste.

MOCVD of CeO2 and SiO2 Mixture Films Using Alkoxy Sources

Mixed oxide compound films of CeO2 and SiO2 were deposited by MOCVD using Ce(OCEt2Me)4 and TEOS (tetraethyl orthosilicate) as the source materials. Oxide film deposition occurred only with the intermittent introduction of TEOS source. The XPS analyses revealed that Si existed mainly as the silicate phase and uniformly distributed along the depth in the CeO2 film. The amount of Si in the deposited film was not affected by the duration time of TEOS introduction. From the results of XRD and XTEM, the crystallization of the CeO2 film was suppressed by mixing SiO2 even after the post-annealing at 500°C.

The effect of metallic oxide deposition on the electrochemical behaviour of Al-Zn-Mg-Sn alloy in natural tropical seawater

The potential of aluminium alloys as anode materials in cathodic protection system has been explored and a significant improvement has been achieved. However, for marine application, it is quite difficult to maintain continuous activation process due to passivation behavior of aluminum alloys. Therefore, to choose the best activation mechanism for aluminium alloy in marine environment, it has to be considered from various points such as alloy composition and surface treatment. This paper report the effect of metallic ruthenium oxide (RuO2) deposition on the surface of as-cast Al-Zn-Mg-Sn alloy and to study the effect of its presence on the electrochemical behavior using direct current (DC) electrochemical polarization and current capacity measurement. The morphology and topography of corroded surface were studied by the aid of scanning electron microscope (SEM) and confocal laser scanning microscope (CLSM) respectively. Results from this study showed that the presence of intermetallic compound (Mg2Sn) and also mixed metal oxide compound (Al2O3 and RuO2) on the alloy surface has been very useful in improving electrochemical reaction and charge transfer activities in chloride containing solution. This study also showed that RuO2 catalytic coating applied on the surface of Al-Zn-Mg-Sn alloy has slightly increased the corrosion current density compared to Al-Zn-Mg-Sn without RuO2. The corrosion morphology and topography of corroded surface of Al-Zn-Mg-Sn alloy deposited with RuO2 was found more uniform corrosion attack with the formation of porous and fibrous mud-like crack on outer layer. Based on surface morphology and 3D topographic studies, these features were believed to facilitate ionic species adsorption and diffusion through corrosion product layer at solution-alloy interface. Deposited RuO2 films also was found to increase of current efficiency by more than 10%.

Thorium fuel performance assessment in HTRs

Thorium as a nuclear fuel is receiving renewed interest, because of its widespread availability and the good irradiation performance of Th and mixed (Th,U) oxide compounds as fuels in nuclear power systems. Early HTR development employed thorium together with high-enriched uranium. After 1980, most HTR fuel systems switched to low-enriched uranium. After completing fuel development for AVR and THTR with BISO coated particles, the German program expanded efforts on a new program utilizing thorium and high-enriched uranium TRISO coated particles for advanced HTR concepts for process heat applications (PNP) and direct-cycle electricity production (HHT). The combination of LTI inner and outer pyrocarbon layers surrounding a strong, stable SiC layer greatly improved manufacturing conditions and the subsequent contamination and defective particle fractions in production fuel elements. In addition, this combination provided improved mechanical strength and a higher degree of solid fission product retention, not known previously with HTI-BISO coatings. The improved performance of the HEU (Th,U)O2 TRISO fuel system was successfully demonstrated in three primary areas of development: manufacturing, irradiation testing under normal operating conditions, and accident simulation testing. In terms of demonstrating performance for advanced HTR applications, the experimental failure statistic from manufacture and irradiation testing are significantly below the coated particle requirements specified for PNP and HHT designs at the time. Covering a range to 1300°C in normal operations and 1600°C in accidents, with burnups up to 13% FIMA and fast fluences to 8×1025m−2 (E>16fJ), the results exceed the design limits on manufacturing and operational requirements for the German HTR Modul concept, which were: <6.5×10−5 for manufacturing; <2×10−4 for normal operating conditions; and <5×10−4 for accident conditions. These performance statistics for the HEU (Th,U)O2 TRISO fuel system are in good agreement with similar results for the LEU UO2 TRISO fuel system.

Americium-based oxides: Dense pellet fabrication from co-converted oxalates

Mixed oxides are used as nuclear fuels and are notably envisaged for future fuel cycles including plutonium and minor actinide recycling. In this context, processes are being developed for the fabrication of uranium–americium mixed-oxide compounds for transmutation. The purpose of these processes is not only the compliance with fuel specifications in terms of density and homogeneity, but also the simplification of the process for its industrialization as well as lowering dust generation. In this paper, the use of a U0.85Am0.15O2±δ powder synthesized by oxalate co-conversion as a precursor for dense fuel fabrications is assessed. This study notably focuses on sintering, which yielded pellets up to 96% of the theoretical density, taking advantage of the high reactivity and homogeneity of the powder. As-obtained pellets were further characterized to be compared to those obtained via processes based on the UMACS (Uranium Minor Actinide Conventional Sintering) process. This comparison highlights several advantages of co-converted powder as a precursor for simplified processes that generate little dust.

Topochemical modifications of mixed metal oxide compounds by low-temperature fluorination routes

AbstractIn this review, we discuss recent developments in the use of low-temperature fluorination routes for the topochemical modification of mixed metal oxide compounds. By applying such methods, material properties (such as magnetism, superconductivity, electrical conductivity, and ionic conductivity) can be tuned in a wide range. Furthermore, oxide fluoride compounds are interesting from a structural point of view, and while differentiating between oxide and fluoride ions has proved to be difficult using diffraction methods, strategies (e.g., bond valence sum calculations) to overcome this problem have been shown to be possible. In addition, this review concludes with an outlook on future prospects in the field of oxide fluoride compounds.

AlN thin films prepared by ArF plasma assisted PLD. Role of process conditions on electronic and chemical–morphological properties

Aluminium nitride thin films were deposited on n-Si substrates by RF plasma activated reactive pulsed laser deposition (PLD). An ArF excimer pulsed laser, 10 Hz and 2.5 J/cm2 energy fluence, has been used to ablate a pure Al target in a reactive atmosphere of N2 plasma (generated by a RF source), at varying processing parameters (substrate temperature, time, and N2 plasma configuration). We studied the dependence and correlation of structural and electronic properties with the experimental conditions. The chemical composition of deposited material has been determined by both Raman and X-ray photoelectron spectroscopy (XPS). Electrical resistivity has been evaluated by the sheet resistance method. Both spectroscopic characterizations (Raman and XPS) show a strong dependence in the formation of AlN on the deposition temperature. At low temperatures, there is little formation of nitride, with a prevalence of aluminium oxide, while at higher temperatures the N uptake increases, with AlN formation. Raman analysis also highlights the formation of nano-structures, for temperatures ≥400∘C. These material characteristics have a fundamental influence on the electronic properties. Indeed, electrical resistivity properties have been found to be strongly dependent on the film structure, nitrogen incorporation, and presence of mixed oxide compounds, closely related to deposition temperature.

Li-storage and cycling properties of Sn–Sb-mixed oxides, (M1/2Sb1/2Sn)O4, M = In, Fe

The mixed oxide compounds, (M1/2Sb1/2Sn)O4, M = In and Fe are prepared by the high-temperature solid-state reaction, at 800 °C for M = In and at 1,150 °C for M = Fe. High-energy ballmilling is used to reduce the particle size to nm-range. The compounds are characterized by X-ray diffraction, Rietveld refinement, scanning electron microscopy, and Brunauer–Emmett–Teller surface area methods. The Li-storage and cycling properties of the bare and ballmilled compounds are evaluated by galvanostatic cycling at ~0.15 C and in the voltage ranges 0.005–1.0 and 0.005–1.2 V vs. Li up to 50 (or 100) cycles and by cyclic voltammetry (CV) at room temperature. Effect of electrode heat treatment and carbon nanotube (CNT) addition is also studied. Initial reversible capacities in the range 425–550 mAh g−1 are observed depending on the metal (M) upper cut-off voltage, CNT content and electrode heat treatment. Ballmilled (In1/2Sb1/2Sn)O4 showed a stable capacity of 445 mAh g-1 up to 30 cycles and 5 % capacity fading after 50 cycles. In all other cases, capacity fading is observed ranging from 9 to 60 %. The CV showed that the main cathodic and anodic peaks occur at 0.15–0.25 V and ~0.5 V vs. Li, respectively, for both M. The reaction mechanism involves alloying–de-alloying reactions of Sn and In with Li3Sb or Fe acting as conducting matrix, and corroborated by the ex-situ X-ray diffraction data on (In1/2Sb1/2Sn)O4.

Anomalous Surface Compositions of Stoichiometric Mixed Oxide Compounds

Coated: Surface analytical studies (including low-energy ion scattering (LEIS)) show that the outer surface of bulk, stoichiometric mixed vanadates and molybdates can be strongly enriched with VOx or MoOx species. Such surface reconstruction even in the calcined initial state has implications for the discussion of catalytic and other materials properties. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Microwave-assisted solid-state decomposition of La[Co(CN) 6]·5H 2O precursor: A simple and fast route for the synthesis of single-phase perovskite-type LaCoO 3 nanoparticles

Pure and single-phase nanoparticles of perovskite-type LaCoO 3 were prepared via microwave-assisted solid-state decomposition of La[Co(CN) 6]·5H 2O precursor in the presence of CuO powder as a strong microwave absorber within a very short reaction time of 10 min. Product was characterized by X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FT-IR), Raman spectroscopy, UV–visible spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and surface area measurement. The method is simple, fast and energy efficient and resulted in fine particles (10–30 nm) with high specific surface area and narrow size distribution. This hybrid microwave heating route is promising for the synthesis of other mixed oxide and related compounds.

Microwave assisted synthesis of manganese mixed oxide nanostructures using plastic templates

The synthesis method for obtaining sub-micrometric structures of rare earth manganese-based mixed oxide compounds is described. Pore wetting of porous polycarbonate templates with the liquid precursor was followed by a two-stage thermal treatment to obtain single phase La 0.325Pr 0.300Ca 0.375MnO 3 hollow and solid structures, with external diameter determined by the sacrificial template pore size. The first thermal stage, a microwave assisted denitration process, determines the shape of the structures. The second treatment, performed at 1073 K, allows to obtain the crystallographic structure of the compound. A variety of techniques (scanning and transmission electron microscopy, scanning probe microscopy) allowed to fully characterize the microstructure and morphology of these self-standing manganite nanostructures. For 1 μm pore size templates we obtained tubes, with external diameter around 800 nm and wall thickness around 150 nm; densely packed nanoparticles sized 20–50 nm are the building blocks of the walls. For pore size below 0.1 μm, solid nanowires were obtained, the size of constituent crystallites being around 10 nm. Overall obtained material exhibits ferromagnetic ordering below 200 K.

Tetradecanol dehydrogenation over CuO catalyst supported on BaO, CaO and MgO

Bimetallic Cu/Mg, Ca, Ba mixed oxide compounds acted as catalyst in the dehydrogenation of tetradecanol into tetradecanal, and were characterized by thermal gravimetric analysis, surface area, X-ray diffraction, temperature-programmed reduction and desorption of CO2. Alkaline earth oxides increased the total basic site concentration in the order of Mg>Ca>Ba. A correlation was found between the total basic site concentration and initial dehydrogenation rate.

A theoretical analysis on electronic structure of the (110) surface of TiO 2–SnO 2 mixed oxide

Mixed oxide compounds, such as TiO 2–SnO 2 system are widely used as gas sensors and should also provide varistor properties modifying the TiO 2 surface. Therefore, a theoretical investigation has been carried out characterizing the effect of SnO 2 on TiO 2 addition on the electronic structure by means of ab initio SCF-LCAO calculations using all electrons. In order to take into account the finite size of the cluster, we have used the point charge model for the (TiO 2) 15 cluster to study the effect on electronic structure of doping the TiO 2 (110) surface. The contracted basis set for titanium (4322/42/3), oxygen (33/3) and tin (43333/4333/43) atoms were used. The charge distributions, dipole moments, and density of states of doping TiO 2 and vacancy formation are reported and analysed.

Role of bond coordination and molecular volume on the dielectric constant of mixed-oxide compounds

First-principles calculations have been employed to study clusters of Zr embedded in SiO2. Stable complexes are found with four, six, and seven oxygens coordinated to the Zr atom. Consistent with experiment, the higher coordinated complexes are the most stable. These also have a higher density, and hence, smaller molar volume. This smaller molar volume provides an explanation of the increased dielectric constant of ZrxSi1−xO2 mixed-oxide systems for small amounts of Zr (x&amp;lt;0.3). An unusual sevenfold coordinated structure is described.

Anodic oxidation as a low thermal budget process for passivation of SiGe

The oxide composition and the electronic behavior of anodically and thermally oxidized SiGe single crystals and epitaxial layers have been investigated. X-ray photoelectron spectroscopies show the existence of Si4+ and Ge4+ only in the anodic oxides, whereas the thermally grown oxide layers on SiGe consist of pure SiO2. Infrared spectroscopy reveals the existence of Ge as part of mixed oxide compounds like Si–O–Ge and not as GeO2 up to a Ge content of 26 at.% in the SiGe lattice. Ge segregation at the oxide/SiGe interface and relaxation phenomena of the strained SiGe lattice of epitaxial layers, which occur during thermal treatments, are completely suppressed using the anodic treatment. In addition, the defect concentration at the interfaces is reduced as obtained from photoluminescence and capacitance–voltage measurements as compared to thermally grown oxides. The amount of Ge in the lattice has an influence on the defect concentration.